Laser diodes are used in many laser applications, such as in laser printing, optical data storage, long-haul fiber communication, spectroscopy, metrology, barcode scanners, and fiber amplifier pump sources. Laser diode structures generally include a central waveguide/quantum well “active” region flanked by upper and lower cladding layers. This configuration is also known as separate confinement heterostructure (SCH). Because of its high refractive index, light is confined to this active region “core” of the structure, where optical gain is produced. The upper and lower cladding layers are formed from material having a lower refractive index than the core, and serve to contain the optical mode. This arrangement guides the optical mode along the active region core, creating a laser light beam that is emitted from a face of the structure.
FIG. 7 is a simplified perspective view depicting a conventional Indium-Gallium-Nitride (InGaN) multi-quantum-well (MQW) laser diode 50, which is exemplary of the type of nitride-based laser diode specifically addressed by the present invention. Referring to the lower portion of FIG. 7, laser diode 50 includes an n-doped layer 62 (e.g., Silicon-doped Gallium-Nitride (GaN:Si)) formed on a substrate 60 (e.g., Sapphire (Al2O3), Silicon-Carbide (SiC), Aluminum-Nitride (AlN), or Gallium-Nitride (GaN)). An n-electrode 64 (e.g., a Titanium-Aluminum (Ti/Al) or Titanium-Gold (Ti/Au) bilayer) and an n-doped cladding layer 66 (e.g., Si-doped Aluminum-Gallium-Nitride (AlGaN:Si)) are formed on n-doped layer 62. A stack is formed on n-doped cladding layer 66 that includes an n-doped waveguide layer 68 (e.g., GaN:Si), a multiple quantum well (MQW) region 70 including multiple (e.g., three) InGaN quantum wells separated by GaN barrier layers, a p-doped waveguide layer 74 (e.g., GaN:Mg), a p-doped cladding layer 76 (e.g., AlGaN:Mg), a second p-doped contact layer 78 (e.g., GaN:Mg), and a p-electrode 80 (e.g., a Ni/Au bilayer).
During the operation of InGaN MQW laser diode 50, a suitable voltage potential is applied to n-electrode 64 and p-electrode 80. The respective n-type and p-type materials inject electrons and holes from these electrodes to the p-n junction provided by MQW region 70, which produces a highly coherent (in this case blue) laser beam LB that is emitted from an area 51 located on a face of laser diode 50. In general, the purpose of the waveguide and cladding layers is to confine the optical mode to a central (core) region of MQW region 70 associated with area 51. This is achieved by forming waveguide layers 68 and 74 from materials having relatively high refractive indexes (although lower than that of MQW region 70), and cladding layers 66 and 76 from materials having relatively low refractive indexes. For several reasons, cladding layers 66 and 76 are formed by adding Al to the material used to form waveguide layers 68 and 74, along with an appropriate dopant (e.g., Si or Mg).
It is critical that laser diodes be precisely formed and made from materials of excellent structural and optoelectronic quality in order to optimize the emitted laser beam. Structural defects (such as dislocations or cracks) or impurities can seriously degrade the luminescence efficiency of the semiconductor materials. In addition, the thickness and shape of the various layers are important to optimize the emitted laser beam.
A problem associated with the use of AlGaN:Mg to produce upper cladding layer 76 of InGaN laser diode 50 (FIG. 7) is that both the thickness and Al concentration are limited by cracking, which then limits the refractive index difference between upper cladding layer 76 and waveguide layer 74 and the active region formed by MQW region 70, and consequently limits the resulting optical confinement. In addition, it is difficult to achieve a high hole concentration in AlGaN alloys because the ionization energy of Mg acceptors increases with increasing Al content. Therefore, Mg doped AlGaN cladding layers increase the series resistance, and ultimately produce undesirable heating of InGaN laser diode 50 during operation. Furthermore, reliable control of the lateral optical mode has proven to be difficult in conventional InGaN laser devices, where typically a ridge-waveguide structure is employed for lateral confinement of the optical mode. A lateral index step is achieved by dry-etching upper AlGaN:Mg cladding layer 76. Because there is no reliable selective etch process known for AlGaN, the etch depth and the resulting index step are difficult to control.
FIGS. 8(A) and 8(B) are graphs depicting modeling data associated with conventional nitride laser diode structure 50 having a ten-InGaN/GaN MQW region 70 and an upper cladding layer 76 comprising Al0.08Ga0.92N:Mg. FIG. 8(A) is a graph showing calculated confinement factors Γ depending on the AlGaN:Mg cladding layer thickness, and indicates a confinement factor Γ of the optical mode is expected to be around 0.08. FIG. 8(B) is a graph showing calculated metal absorption loss for a conventional 10 InGaN/GaN-MQW laser diode structure for different top p-electrode metal layers as a variation of the AlGaN:Mg cladding layer thickness. As indicated, the absorption loss from the top p-metal layer in such a structure depends greatly on the thickness of the AlGaN:Mg cladding layer. In order to achieve reasonable low loss values (˜1 cm−1), a cladding layer thickness in the range of 400–500 nm is necessary. Further, these loss values are only a lower estimate, not taking into account the losses due to surface roughness (e.g., induced by the metal alloy fabrication process) or metal penetrating into the (Al)GaN layers. Therefore, in order to improve the optical confinement and reduce absorption loss, the AlGaN:Mg layer ideally should be kept as thick as possible. However, as set forth above, AlGaN:Mg contributes significantly to the series resistance of the device, and also has to be kept thin enough in order to avoid cracking, which degrades the optoelectronic quality of the laser diode. An AlGaN:Mg free device structure would therefore be beneficial in order to overcome these two problems.
What is needed is an index guided single-mode laser diode structure that does not include a p-AlGaN cladding layer, while still maintaining the same optical confinement factor and avoiding significant absorption loss in the p-metal. In addition, the series resistance in the laser diode should be largely reduced.